Python Feature Slice Architecture

"Organize around business capabilities, not technical layers." -- Sam Newman, Building Microservices

Core Philosophy

Feature slice architecture organizes code by business capability, not technical layer: instead of controllers/, services/, repositories/, you have features/orders/, features/users/ — each a self-contained vertical slice. In Python with FastAPI, the mediator pattern (FreeMediator in .NET) is replaced by FastAPI's native Depends(); service classes are injected directly into route handlers. CQRS separation is a structural/naming convention — OrderReadService vs OrderWriteService — not a library contract.

Non-Negotiable Constraints:

1. FEATURE ISOLATION — no cross-feature imports; features communicate only through shared/ domain models.
2. SERVICE OWNS LOGIC — routers are thin (extract → call service → return); no business logic in handlers.
3. PYDANTIC V2 EVERYWHERE — all request/response types are Pydantic models with response_model set; no bare dict returns.
4. PROTOCOL INTERFACES — typing.Protocol for service interfaces so tests run without concrete implementations.
5. ASYNC-FIRST — all I/O uses async def; no blocking calls in async context (asyncio.to_thread() if unavoidable).

Full principle table, KB lookups, discipline rules, anti-patterns, and error recovery live in references/conventions.md.

Workflow

DETECT     Identify existing structure (flat/layered/feature-based), the FastAPI entry point
           (main.py/app.py), the router registration pattern, the ORM setup, and the auth pattern.
           (grep APIRouter/include_router; find main.py/app.py.)

SCAFFOLD   Create the feature package:
             features/<name>/{__init__.py, router.py, service.py, models.py, dependencies.py}
             tests/features/<name>/{test_router.py, test_service.py}
           (File-by-file content in references/feature-folder-template.md; read/write split in cqrs-conventions.md.)

REGISTER   Wire the router: app.include_router(<name>_router, prefix="/api/v1", tags=["<name>"]).

VERIFY     uvicorn starts; /docs renders the endpoints; pytest tests/features/<name>/ passes;
           grep confirms no cross-feature imports.

Exit criteria: feature package created with thin router, Protocol-typed read/write services, Pydantic v2 models, and tests; router registered under /api/v1; app starts, /docs renders, tests pass, and no cross-feature imports exist.

State Block

<python-feature-slice-state>
phase: DETECT | SCAFFOLD | REGISTER | VERIFY | COMPLETE
feature_name: [name]
existing_structure: flat | layered | feature-based | unknown
app_entry_point: [file path]
router_registered: true | false
tests_scaffolded: true | false
read_service_created: true | false
write_service_created: true | false
last_action: [description]
next_action: [description]
</python-feature-slice-state>

Output Template

• Scaffold checklist, feature folder structure — references/output-templates.md.
• File-by-file scaffold content (router, service, models, dependencies, tests) — references/feature-folder-template.md.
• Read/write service separation conventions — references/cqrs-conventions.md.
• Principle table, KB lookups, discipline rules, anti-patterns, error recovery — references/conventions.md.

Integration with Other Skills